Dark Teplar/ Dark Archon problems?

i have a gateway a ton of pylons only 31 out of 121 units a nexus a cathedral Templar archives and 2 dark Templars and i have selected them but for some reason i cant merge them or find the dark archon meld button and i cant remember how i did it on mt other computer can any one tell me why it wont let me merge them

Other Answers

If you're in the campaign, I believe there are a few missions where you get DT's but can't meld them into Darchons yet. If not, then the only other explanation is that you have other units selected as well, just as AP said.